The invention relates generally to an improvement in an apparatus and procedures for making rapid determination of the strength of metal materials from hardness evaluation of the materials, and particularly to the elimination of errors and uncertainties in conventional hardness testing techniques that relate yield and/or tensile strength to hardness.
Hardness evaluations are commonly employed in quality assurance testing to indicate material strength. Typically, correlations for particular alloy-temper combinations are expressed as equations relating strength (S) to an observed hardness number (H) in the form of EQU S=a+b.times.H (1)
where the values of a and b depend on the hardness test scale selected, i.e., on the conditions of the hardness test.
Small scale tests, such as hardness tests, are convenient and less expensive than tests involving machined or otherwise specially prepared specimens for tensile testing. However, such convenience has led to a bewildering variety of hardness test scales. Generically, these can be divided into scales using ball-shaped indentors such as the Brinell and Rockwell indentors or diamond-shaped indentors such as Vickers or Knoop.
One significant disadvantage of the Rockwell test lies in the large variety of scales, i.e, no single scale adequately spans the whole range of interest for aluminum alloys, for example. Each scale has an optimal application in terms of material strength (temper) and minimum thickness necessary to avoid the so-called anvil effect, which effect involves variations in hardness readings due to the hardness of the structure supporting the specimen.
Nonetheless, for approximate practical purposes, the Rockwell scales can be ranked in terms of their severity of loading by dividing the applied measured pressure or load (L) by the diameter of the ball squared (D.sup.2), which is the ratio L/D.sup.2. If the reading on a particular scale is above say 100 (a hardness number), one has to then provide a scale with lower numerical readings; if the normal scale produces low readings, such as values below 20, a scale with a lower L/D.sup.2 value is needed. This often necessitates changing scales in the midst of an investigation, which further complicates the use of a strength hardness equation such as equation (1) above.
Rockwell and other hardness tests, in addition, do not provide unambiguous predictions of yield or tensile strength of materials tested. This is the result of the influence of work hardening that occurs in the process of making the impressions. This influence can be understood by expressing hardness as a flow stress and relating it to yield or tensile strength through the well-known, constitutive stress-strain relationship.
The general conclusion from such analyses has been that one must know the work-hardening coefficient and the degree of strain imparted by the indentation process to predict yield or tensile strength from a hardness test.
An empirical way of circumventing the need for such complete knowledge was proposed in an article entitled "Estimating Yield Strength from Hardness Data" by Robert A. George, Subimal Dinda and Arthur S. Kasper, published in the May 1976 issue of Metal Progress. The authors use the basic relationship between applied force or load (L), indentor diameter (D) and the impression (d) of the form ##EQU1## to predict yield strengths of various steels. (A and m are empirical constants discussed in detail hereinafter.) This work correlated yield strength with the constant A in the form of a regression equation, i.e., EQU ys (ksi)=0.325A (3)
with A being determined from a nomographic solution to equation (2) with particular Rockwell numbers. The A value, as determined by such a method, is the solution to equation (2) with d/D being equal to 1.0 (one); here A is only a single value that is employed to estimate the yield strength of the metal tested from an empirical relationship, i.e., equation (3). In this work, there is no indication of the tensile strength of the material.
In a paper entitled, "Flow Property Measurements from Instrumented Hardness Tests" by P. Au, G. E. Lucas, J. W. Sheckerd and G. R. Odette, published in 1980 by the American Society for Metals, a procedure similar to the above George et al paper correlates flow property information developed from instrumented hardness tests with true plastic strain of the samples tested.
However, the disclosure by Au et al requires certain assumptions concerning interfacial pressure (Pm) and axial flow of the specimen (.sigma..sub.t), as discussed on pages 600 and 601 of the article and formulated by the equations PM=2.8.sigma..sub.t and ##EQU2## The applicant's approach does not require these assumptions.
In addition, Au et al do not specifically show that hardness type measurements yield engineering tensile and/or yield strength estimates, as disclosed and taught by Applicant.
And lastly, it should be noted that the Au et al strain range is only about 0.01&lt;.epsilon.p&lt;0.07 (FIGS. 3-7 of the article), which corresponds to ##EQU3## i.e., a much smaller range of hardness than that covered by Applicant's approach, as discussed below.